Near-infrared vibrational modes

Circular dichroism is always a small fraction of the direct absorption. Using visible, UV or infrared ratios as a guide we would expect that the ratio will be — aiX THz, where a is of the order of the atom or ion spacing. This is a very small number at terahertz frequencies. But the molecular unit, terahertz chromophore that is responsible for the terahertz absorption involves many atoms and ions. For a helix 7 periods in length, the long wavelength acoustic mode in our simple model is — 140 and we expect to recover circular dichroism/absorption ratios that are reasonable relative to those of electronic excitations or near infrared vibrations. For this particular, rather... 

Near-infrared (NIR), the spectral region between 780 and 3000 nm, is characterized by broad and overlapping spectral peaks produced by the overtones and combinations of infrared vibrational modes. Figure 28.5 shows typical NIR absorption spectra of fat, water, and starch. Exploitation of this spectral region for in vivo analysis has been hindered by the same complexities of nonpulsatile tissue spectroscopy described above and is further confounded by the very broad and indistinct spectral features characteristic of the NIR. Despite these difficulties, NIR spectroscopy has garnered considerable attention, since it may enable the analysis of common analytes. 

The CO2 laser is a near-infrared gas laser capable of very high power and with an efficiency of about 20 per cent. CO2 has three normal modes of vibration Vj, the symmetric stretch, V2, the bending vibration, and V3, the antisymmetric stretch, with symmetry species (t+, ti , and (7+, and fundamental vibration wavenumbers of 1354, 673, and 2396 cm, respectively. Figure 9.16 shows some of the vibrational levels, the numbering of which is explained in footnote 4 of Chapter 4 (page 93), which are involved in the laser action. This occurs principally in the 3q22 transition, at about 10.6 pm, but may also be induced in the 3oli transition, at about 9.6 pm. 

The above picture points to the very interesting possibility of selectively inducing or enhancing the polymerisation process, at a temperature where this is unlikely, by resonantly driving with an intense laser beam in the infrared the vibrational modes and wc that are involved in the polymerisation. As a consequence of their anharmonicity (45) these modes, when driven near resonance by an electromagnetic field, beyond a certain critical value of the later, can reach amplitudes comparable to the critical ones required for the polymerisation to be initiated or proceed the anharmonicity in the presence of the intense laser beam acts as a defect and localizes the phonons creating thus a critical distorsion. 

Infrared light is divided into three zones the near-infrared, the mid-infrared, and the far-infrared, as shown in Table 12.2. The wavelength of the near-infrared is below 2.5 pm, that of the mid-infrared ranges from 2.5 pm to 25 pm, and that of the far-infrared is above 25 pm. Infrared emission between 3 pm and 30 pm is caused by vibrational modes of the molecules, while that above 30 pm is caused by rotational modes. 

Most SHG studies involve incident energies in the visible or near-infrared spectrum. Infrared SHG studies are hindered by the current lack of sufficiently sensitive IR detectors. However, the sum frequency generation (SFG) technique allows one to obtain surface-specific vibrational spectra. In SFG, two lasers are focused on the sample surface, one with a fixed frequency in the visible and one with a tunable range of IR frequencies. The sample surface experiences the sum of these frequencies. When the frequency of the infrared component corresponds to a molecular vibrational mode, there is an increase in the total SHG signal, which is detected at the visible frequency [66]. The application of such... 

For any vibrational mode, the relative intensities of Stokes and anti-Stokes scattering depend only on the temperature. Measurement of this ratio can be used for temperature measurement, although this application is not commonly encountered in pharmaceutical or biomedical applications. Raman scattering based on rotational transitions in the gas phase and low energy (near-infrared) electronic transitions in condensed phases can also be observed. These forms of Raman scattering are sometimes used by physical chemists. However, as a practical matter, to most scientists, Raman spectroscopy means and will continue to mean vibrational Raman spectroscopy. 

The term unidentified infrared emission is used to refer to the long-known emission features of interstellar dusts in the spectral region from just over 3,000 cm-1 to below 800 cm-1 (Gillett et al. 1973). These features comprise sharp IR bands at 2,920,1,610, and 880 cm-1, as well as a broader envelope near 1,300 cm-1. In addition, a recurrent mode at 3,050 cm-1, a weak mode near 1,450 cm-1, and a shoulder near 1,150 cm-1 are observed. These spectral features can all be attributed to vibrational modes of hydrogenated carbon species, as summarized in Table 2.1. The chemical structure of these species remains the subject of debate. Furthermore, a number of carbon-rich astronomical objects reveal an emission feature in the far-IR at 490 cm-1, of unclear attribution (Kwok et al. 1989). 

Vibrational Spectroscopy [Infrared (mid-IR, NIR), Raman]. In contrast to X-ray powder diffraction, which probes the orderly arrangement of molecules in the crystal lattice, vibration spectroscopy probes differences in the influence of the solid state on the molecular spectroscopy. As a result, there is often a severe overlap of the majority of the spectra for different forms of the pharmaceutical. Sometimes complete resolution of the vibrational modes of a particular functional group suffices to differentiate the solid-state form and allows direct quantification. In other instances, particularly with near-infrared (NIR) spectroscopy, the overlap of spectral features results in the need to rely on more sophisticated approaches for quantification. Of the spectroscopic methods which have been shown to be useful for quantitative analysis, vibrational (mid-IR absorption, Raman scattering, and NIR) spectroscopy is perhaps the most amenable to routine, on-line, off-line, and quality-control quantitation. 

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